Ultrasound refers to any sound wave or vibration whose frequency exceeds the highest threshold of 20kHz (kHz) that can be heard by the human ear. It is a sound wave with a frequency higher than 20000 Hz. It has good directivity, strong reflection ability, and it is easy to obtain more concentrated acoustic energy. It travels farther in water than air, and can be used for distance measurement, speed measurement, cleaning, welding, and breaking. Stone, sterilization, etc. Ultrasound is widely used in many fields such as medicine, military, industry, agriculture, intelligence, etc. due to its high frequency characteristics. Ultrasound is named because its lower frequency limit exceeds the upper limit of human hearing.
Scientists call the number of vibrations per second the frequency of sound, and its unit is Hertz (Hz). The frequency range of sound waves that our human ears can hear is 20Hz-20000Hz. Sound waves with a frequency higher than 20,000 Hz are called "ultrasonic waves". The frequency of ultrasonic waves commonly used in medical diagnosis is 1 MHz to 30 MHz.
Some animals, such as dogs, dolphins, and bats, have ears beyond humans, and therefore can hear ultrasound waves. Others have used this feature to make dog flutes that can generate ultrasonic waves to call dogs.
The so-called ultrasonic waves only pass through elastic and inertial media, such as air. When the air itself expands or compresses, there is wave propagation through the movement of its molecules. Therefore, sound waves cannot propagate in a vacuum. Human hearing can perceive fluctuations and calls it sound. At this time, sound waves are called audible waves.